BUG: remove color table entries from mrml file

Rely on the file written to disk by the storage node, 
reduces the size of the mrml file.
Keep the number of colors in the mrml file so that the
table is properly initialised.

Issue #2486


git-svn-id: http://svn.slicer.org/Slicer4/trunk@22344 3bd1e089-480b-0410-8dfb-8563597acbee